Density Functional Theory

Kohn and Hohenberg showed, in 1964, that all the properties of a molecular system can be determined from the electron density function, ρ, of the system.[27]
The energy of a system is a function of the density function, often called a functional:

(9.1)

This means the ground state Hamiltonian can be derived from the density function. Coupled with the Kohn-Hohenberg variational theorem, it is possible to determine the energies and densities of a molecular system, by virtue of the fact that the true ground state electron density minimises the energy functional, just as the true ground state wavefunction minimises the Schrödinger energy.
Density functional theory (DFT) also accounts for electron correlation effects. This, and the fact that it is computationally only marginally more time consuming than a good Hartree-Fock calculation (which does not include electron correlation effects), has led to widespread use of DFT. However, DFT is not strictly an ab initio method, as it does not directly solve the Schrödinger equation. It is included because of its popularity, and it is certainly a valid method, but some people are reluctant to regard it as an ab initio method. The fact that it uses an approximate functional means that DFT can sometimes yield energy values below the true ground state energy. It is also only accurate for ground state symmetries, although work has been done on extending its use for excited states.
Kohn was awarded half of the Nobel Prize for Chemistry in 1998 for his work on density functional theory.
An in-depth discussion of density functional theory is also given in Levine.[7]